Unraveling the pre-metamorphic cooling history of the Koraput Alkaline Complex, India: constraints from feldspar exsolution texture

Intracrystalline exsolution textures in alkali feldspar are common in lithotypes from many alkaline complexes of the Eastern Ghats Granulite Belt (EGGB), India. However, the parentage of these textures and their compositional evolution is not well documented from this granulite belt. This study on the Koraput Alkaline Complex (KAC) in the EGGB documents the exsolution textures from several lithologies, establishes their igneous origin and finally links their compositional modifications with the evolutionary history of the complex. The studied exsolution textures belong to both perthite and mesoperthite. To estimate the temperature of formation of these textures, we used both two-feldspar thermometry, and one-feldspar thermometry following several models. In two-feldspar thermometry, compositions of exsolved alkali feldspar and the adjacent plagioclase feldspar pairs were used. In one-feldspar thermometry, the reintegrated compositions of exsolved alkali feldspars were used. The maximum temperature of formation of exsolution lamellae estimated from two-feldspar thermometry for mesoperthites in nepheline syenite is > 841 °C, and for perthites is > 759 °C, at 7 kbar pressure. Compositions of initially formed plagioclase feldspar lamellae and the host feldspar were more orthoclase rich and more albite rich respectively compared to the observed compositions. Using one-feldspar thermometry the calculated temperatures for alkali gabbro, syenite and alkali feldspar granite are > 870 °C, > 810 °C and > 730 °C, respectively. Compositions of alkali feldspars immediately before exsolution in these rocks were also estimated. Albite and orthoclase contents were nearly equal in mesoperthites; on the other hand, orthoclase content was higher than albite in perthites. Previous studies assigned their thermometric estimation with the minimum temperature of metamorphism that the KAC experienced, but the Ultra High Temperature (UHT) record obtained from the feldspar thermometry of the present study is difficult to correlate with these metamorphic events. Instead, these high temperatures may represent an igneous condition, which remained unaffected throughout the later metamorphic event as documented from nepheline syenite.